This invention relates to the casting of metal strip by continuous casting in a twin roll caster.
In a twin roll caster molten metal is introduced between a pair of counter-rotated horizontal casting rolls that are cooled so that metal shells solidify on the moving roll surfaces and are brought together at a nip between them to produce a solidified strip product, delivered downwardly from the nip between the rolls. The term “nip” is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or series of smaller vessels from which it flows through a metal delivery nozzle located above the nip, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow.
Further, the twin roll caster may be capable of continuously producing cast strip from molten steel through a sequence of ladles. Pouring the molten metal from the ladle into smaller vessels before flowing through the metal delivery nozzle enables the exchange of an empty ladle with a full ladle without disrupting the production of cast strip.
The casting rolls must be accurately set to properly define an appropriate width for the nip, generally of the order of a few millimeters or less. There must also be some means for allowing at least one of the rolls to move relative to the other casting roll to accommodate fluctuations in strip thickness, particularly during start up.
In the past, one of the casting rolls was mounted in fixed journals or was mounted in supports urged against physical stops. The other casting roll was rotatably mounted on supports that could move outwardly against the action of a resisting force enabling that roll to move laterally to accommodate fluctuations in strip thickness. The resisting force was applied by helical compression springs, or alternatively, pressure fluid cylinder units.
A strip caster with spring resisting force against the laterally moveable roll is disclosed in U.S. Pat. No. 6,167,943 to Fish et al. In that case the resistive springs act between roll supports and a pair of thrust reaction structures, the positions of which can be set by operation of a pair of powered mechanical jacks to enable adjustment of the initial compression of the springs to set initial compression forces. The initial compression forces are generally equal at both ends of the roll. The positions of the roll supports on the moveable casting roll are subsequently adjusted after commencement of casting, so that the gap between the rolls is constant across the width of the nip to produce a strip of constant profile. However, as casting continues the profile of the strip will inevitably vary due to eccentricities in the rolls and dynamic changes due to variable heat expansion and other dynamic effects. U.S. Pat. No. 6,167,943 does not provide strip thickness profile control to suppress thickness profile fluctuations during casting.
U.S. Pat. No. 6,837,301 to Nikolovski, et al. provides for controlling strip thickness profile during casting using sensors positioned downstream of the nip. However, U.S. Pat. No. 6,837,301 discloses strip thickness profile control obtained by enabling one of the casting rolls to move laterally outward from the other casting roll against variable resistive forces. The other casting roll is maintained substantially fixed against an adjustable stop.
There remains a need to improve control over the forces that the rolls apply against the strip irrespective of the variation in thickness profile of the strip during production. An apparatus is disclosed for continuously casting thin steel strip comprising:                (a) a pair of counter-rotatable casting rolls having casting surfaces laterally positioned to form a nip there between through which thin cast strip can be cast, and on which a casting pool of molten metal can be formed supported on the casting surfaces above the nip,        (b) at least one actuator capable of moving laterally each casting roll independently toward and away from a given reference location as desired,        (c) location sensors capable of sensing the location of the casting rolls relative to the given reference location and producing electrical signals indicative of each casting roll position in relation to the given reference location, and        (d) a control system capable of receiving the electrical signals indicative of each casting roll position and causing the actuator to move the casting rolls into desired position relative to the reference location for casting metal strip.        
Each casting roll may be mounted on a roll cassette, and may further include actuators disconnectable from the casting rolls to enable the casting rolls to be changed out without dismantling the actuators.
By independently moving, each casting roll is able to move toward and away from the reference location and the nip between the casting rolls. There are reaction forces on the casting rolls from the cast strip and the movement of the adjacent casting roll, but these reaction forces are forces to which the independent movement of the casting rolls is responsive. Usually separate actuators are provided capable of independently moving each casting roll relative to the given reference location, although with mechanical linkage it may be possible to provide independent movement of the casting rolls with one actuator mechanism. The actuators may also be provided to vary the distance between the casting rolls at each end of the casting rolls independently as desired. In any event, the actuators may be selected from the group consisting of servo-mechanisms, hydraulic mechanisms, pneumatic mechanisms, rotating actuators and magnetostrictive actuators, and be capable of moving the casting rolls independently to vary the distance between each casting roll and the given reference location.
The apparatus for continuously casting strip may also have separate location sensors capable of sensing the position of the casting rolls relative to the given reference location at each end of each casting roll independently.
The apparatus for continuously casting strip may further include force sensors capable of sensing the forces exerted on the strip adjacent the nip and producing electrical signals indicative of forces exerted on the strip. The control system may be also capable of receiving the electrical signals indicative of the sensed forces exerted on the strip and causing the actuator to move the casting rolls responsive to the sensed forces exerted on the strip as desired.
The control system may capable of receiving and combining the separate electrical signals indicative of the sensed forces exerted on the strip from each end of each casting roll, and causing one or more actuators to vary the position of the casting rolls responsive to the combined electrical signals. Alternately or in addition, the control system may be capable of receiving the electrical signals indicative of the sensed forces exerted on the strip and combining the electrical signals from an end of one casting roll with the electrical signals from the corresponding end of the other casting roll and causing one or more actuators to vary the position of the casting rolls responsive to the combined electrical signals. Alternately or in addition, the control system may be capable of receiving the electrical signals indicative of the sensed forces exerted on the strip and combining the electrical signals from opposite ends of one casting roll and causing the actuators to vary the position of the casting rolls responsive to the combined electrical signals.
The apparatus for continuously casting strip may also include profile sensors positioned downstream of the nip capable of sensing the strip thickness profile at a plurality of locations along the strip width and producing electrical signals indicative of the strip thickness profile downstream of the nip, and the control system capable of processing the electrical signals indicative of the strip thickness profile and causing the actuator to move the casting rolls responsive to the electrical signals and further control the thickness profile of the cast strip responsive to the electrical signals indicative of the strip thickness profile.
Further, the apparatus for continuously casting strip may include temperature profile sensors positioned downstream of the nip capable of sensing the strip temperature profile at a plurality of locations along the strip width, and producing electrical signals indicative of the strip temperature profile downstream of the nip. The temperature profile sensors may be positioned to determine the temperatures across the cast strip at segments adjacent the nip or further downstream of the nip, and generate electrical signals corresponding to the strip temperature profile in segments across the strip adjacent the nip. Then, the control system may be capable of processing the electrical signals indicative of the strip temperature profile, and causing the actuators to move the casting rolls and further control the thickness profile of the cast strip responsive to the electrical signals indicative of the strip temperature profile.
Also disclosed is a method of continuously casting metal strip comprising the steps of:                (a) assembling a pair of counter-rotatable casting rolls having casting surfaces laterally positioned to form a nip there between through which thin cast strip can be cast, and on which a casting pool of molten metal can be formed supported on the casting surfaces above the nip,        (b) sensing the location of each casting roll relative to a given reference location and producing electrical signals indicative of each casting roll position in relation to the given reference location,        (c) controlling the location of each casting roll independently responsive to the electrical signals indicative of the position of the casting rolls, and        (d) moving each casting roll independently toward and away from the given reference location as desired.        
Each casting roll may be mounted on a roll cassette, with each casting roll being mounted to be capable of moving toward and away from the nip during casting.
The method may include the step of separately moving each casting roll relative to the given reference location. Alternately or in addition, the method may further include the steps of:                sensing the forces exerted on the strip adjacent the nip at each end of each casting roll and producing electrical signals indicative of force exerted on the strip at each end of each casting roll; and        causing actuators to move each end of each casting roll responsive to the electrical signals indicative of the forces exerted on the strip to vary the distance between each end of the casting roll and the given reference location as desired.        
In the method, the moving step may be performed by one or more actuators. However, generally two or more actuators are desired to independently move the casting rolls in relation to the given reference location. In any event, the actuator or actuators are selected from the group consisting of servo-mechanisms, hydraulic mechanisms, pneumatic mechanisms, rotating actuators, and magnetostrictive actuators, and are capable of moving the casting rolls independently to vary the distance between each casting roll and the given reference location. The moving step may be performed by independently varying the distance between the casting rolls at each end of the casting rolls. Alternately or in addition, the moving step is performed by controlling a force urging each roll against the thin cast strip there between during casting. The method may include the additional step of disconnecting the casting rolls to enable the casting rolls to be changed out without dismantling the actuators.